The present invention relates generally to steam precipitation methods for producing polycarbonate resin powder from polycarbonate solutions in nonaqueous solvent (e.g., methylene chloride). More specifically, the present invention relates to a more energy efficient method of steam precipitation which produces a powder having a relatively lower water content, thereby avoiding the need for a substantial portion of the post dryer capacity that would otherwise be required to produce an acceptably dry polycarbonate powder. Dry powder typically has less than 1% by weight volatiles.
Polycarbonates are a widely used class of thermoplastic materials, which are prized for their superior clarity and physical toughness. One preferred method of producing polycarbonates, which is commonly referred to as the “interfacial method,” comprises reacting phosgene and bisphenol-A in a two-phase system having an aqueous and a nonaqueous phase wherein the nonaqueous phase typically comprises methylene chloride as a solvent. Upon reaction, a solution of polycarbonate in methylene chloride is typically formed. Next, the aqueous phase is typically removed and the methylene chloride phase may be washed to help remove residual salts, catalysts and other impurities.
In the final drying step, the polycarbonate in methylene chloride solution must be converted to a dry polycarbonate powder. This step typically accounts for a large fraction of the total production cost because drying the powder is a very energy intensive process, which typically employs hot gas dryers. These dryers require large amounts of electricity and steam to operate and are maintenance intensive. The required electricity and steam are expensive to produce, and their production involves an environmental cost. The drying step is also problematic because the capacity of interfacial polycarbonate synthesis plants is often limited by their capacity to perform this final drying step.
Steam precipitation is a commonly employed industrial method for converting polycarbonate in methylene chloride solution to wet polycarbonate powder. In steam precipitation, the polycarbonate solution is atomized and sprayed into a flowing steam atmosphere. The steam atmosphere is at a sufficient temperature and pressure to cause methylene chloride to evaporate from the small droplets of polycarbonate solution, forming granular particles, which are conventionally recovered in a downstream precipitation piping loop.
Unfortunately, while steam precipitation is relatively efficient at vaporizing the methylene chloride, the steam itself simultaneously partially condenses on the particles, forming a wet powder. The wet powder typically has a water content of from about (or from) 25 to about (or to) 60 percent by weight. Several downstream dryers are necessary to remove the residual water from the wet powder to produce a dry product powder having an acceptable percent water content (wt.), which is typically less than 1 percent.
In common processes, a solution of polycarbonate dissolved in methylene chloride is fed into a jet where it is introduced into a flow of high temperature steam which causes the methylene chloride to vaporize and allows the polycarbonate to solidify. The design of the jet influences the energy efficiency of the process, as well as the particle size of the final polymer powder. Jet designs that increase energy efficiency or improve the particle size distribution of the product make the overall manufacturing process more desirable.
Conventional jet designs establish a baseline for energy efficiency and particle size distribution of the product powder. Increasing the concentration of the polymer solution or preheating the polymer solution before it enters the jet increases overall energy efficiency. Some have described various improvements in steam ratios, resin concentration, and jet design, all designed to improve energy efficiency and resin particle size distribution.
By the present invention, Applicants have discovered an improved steam precipitation method for producing polycarbonate powder having a relatively lower water content than powder produced by conventional steam precipitation. The improved method enables the production of dry powder using less energy (i.e., steam and electricity) and potentially fewer dryers (with less maintenance).